1. Field of the Invention
The present invention relates to a digital signal reproducing apparatus and, more particularly, to a digital signal reproducing apparatus for reproducing a digital signal recorded by an arbitrary self-clock scheme (e.g., an MFM modulation scheme or a 2-7 modulation scheme) on a recording medium such as a floppy disk, a magnetic card, an optical card, or an optomagnetic medium.
2. Description of the Related Art
Various conventional digital signal recording/reproducing schemes used in a variety of fields described above have been proposed. An arbitrary self-clock scheme has been generally used because it can extract a clock signal from a signal modulated during recording. In reproduction of a digital signal recorded by such an arbitrary self-clock scheme on a recording medium, a countermeasure against variations in reproduction pulse width caused by variations in relative speed between the recording medium and a read head and signal variations and omissions caused by damage and dust on the medium must be implemented to accurately demodulate the recorded digital signal regardless of the types of self-clock schemes.
Conventional digital signal reproducing apparatuses include an analog reproducing apparatus for feeding back a phase difference between a reproduced pulse and a self-oscillated clock pulse to a frequency to synchronize the reproduced pulse with a read pulse according to a PLL (Phase-Locked Loop), and a digital reproducing apparatus for reading pulses one by one using a reproduced pulse as a reference pulse to form a window signal.
The former analog prior art is disclosed in Published Examined Japanese Patent Application No. 57-24700 although the disclosed arrangement partially includes a digital arrangement. In the arrangement of this prior art, a read clock synchronized with a reproduced pulse is obtained by a PLL scheme for counting a phase difference between a reproduced pulse and a self-oscillated clock and feeding back a count value to the frequency of the self-oscillated clock. The reproduced pulse is demodulated by the read clock synchronized with the reproduced pulse.
The latter digital prior art is exemplified by Published Unexamined Japanese Patent Application No. 62-241176. In an arrangement proposed by this prior art, the period of a clock bit at a given moment is extracted from a pulse interval during reproduction at the given moment, a correction clock having a value 1/2 the period of the extracted clock bit as a reference period is generated, and a read window signal is formed on the basis of the correction clock, thereby demodulating a reproduced pulse.
In Published Examined Japanese Patent Application No. 57-24700 disclosing the arrangement wherein the phase difference between the reproduced pulse and the self-oscillated clock is fed back to the frequency of the self-oscillated clock to obtain the read clock synchronized with the reproduced pulse, once the phase of the reproduced pulse is stepped out from that of the read clock, it takes a long period of time to properly phase-lock the read clock with the reproduced pulse. In addition, tracking is delayed when variations in medium speed causing large pulse width variations occur. As a result, the reproduced pulse cannot be accurately demodulated, resulting in inconvenience.
In Published Unexamined Japanese Patent Application No. 62-241176 disclosing the arrangement wherein the period of the clock bit is extracted from the pulse interval corresponding to the reception timing of the pulse to form the demodulation window, the clock period varies upon reception of each pulse. For this reason, when the pulse position is abruptly changed due to variations in speed of the medium, a demodulation error occurs. For example, if the current pulse position is delayed from the normal position, the duration of a pulse preceding the current pulse is increased, and the duration of a pulse succeeding the current pulse is decreased. For this reason, when the clock period is increased in accordance with the preceding pulse, the next pulse does not reach within the window ON period, and accurate demodulation cannot be performed. For example, when the pulse width is abruptly increased by jitter (fluctuation of a one-shot signal) caused by damage and dust on the medium and the next pulse width is decreased, a bit cannot be fitted within the data window. As a result, the clock bit and the data bit are reversed in order to cause a demodulation error.
The latter digital prior art is susceptible to variations by jitter. More specifically, in an apparatus for reproducing a digital signal recorded on a recording medium, jitter caused by variations in read speed of the recording medium easily affects reproduction. At the same time, jitter such as variations in reproduced pulse width caused by damage and dust on the recording medium also adversely affects reproduction. It is, therefore, difficult to accurately demodulate a digital signal (data) modulated and recorded by an arbitrary self-clock scheme.